Externally pressurized bearings include porous carbon, orifice, and step compensated air bearings and orifice, self, diaphragm or other similarly compensated fluid bearings. All these bearings have several things in common and that include: the presence of a pocket region (either large or tiny as in the case of porous graphite) that is connected to an external supply line; the presence of a supply line that connects the bearing up to an external pressure supply; the need to be located with respect to the bearing rail or shaft that they will ride on with great precision; a carriage or housing that they are supposed to support with respect to the housing.
In the past, the surfaces on the carriage that the bearings were attached to were often precision machined such that when the bearing pads were attached to them, the entire assembly could be slid over the bearing rails or a shaft inserted. Needless to say, this was very expensive, and required that every rail assembly be carefully measured, and then a carriage carefully machined and fitted to the rail assembly, so each system was comprised of a matched set.
An alternative means was to use a rough machined carriage that used set screws to push against the backs of the bearing pads which forced them into contact with the bearing rail or shaft. Epoxy was then injected into the gap between the backs of the bearing pads and the rough inner surfaces of the carriage or housing. This method had one serious drawback. The set screws force caused local depressions in the geometry of the bearing pad that caused the bearing gap to be uneven, thereby increasing the sensitivity to a changing gap caused by tolerance errors in the bearing rail or shaft geometry and mounting. This increased the likelihood of a "crash" once the machine became operational. In addition, for ceramic structures, where it is expensive and undesirable to have stress concentrating holes, or in some types of housings, the auxiliary holes produced by this method are unacceptable. In the event that a thick enough bearing pad or enough set screws were used to avoid deforming the pad, desirable results were obtained, but at increased cost.
Other techniques have been proposed to develop a good joint between a bearing block that is in contact with a bearing rail, and a structure. Devitt Machinery of Aston Pa. and Philadelphia Resins of Montgomery Pa. have for many years sold special epoxies that are designed not to shrink and that fill the gap between a bearing block and a structure. The process is commonly used with a linear-guide type bearing, such as shown in U.S. Pat. No. 5,102,235 of common assignee herewith. Matching parallel rails are bolted to a machine bed and carriages are attached to each of the rails. With this design, a carriage would be positioned over the bearing blocks, and then a high effective viscosity non-Newtonian epoxy is injected into the space between the tops of the bearing blocks and the rough underside of the carriage. In this manner, the carriage may be aligned with respect to the bearings and secured in place without requiring any precision machining of the carriage-to-bearing mounting surface. Set screws are typically used to adjust the carriage position prior to injecting the epoxy.
U.S. Pat. No. 4,626,299 of Knight et. al. describes a similar means for attaching guideways to a machine tool bed, filling the space between them and the machine tool with bonding material, although as shown has been done also by others in everyday practice. In U.S. Pat. No. 4,726,103 a system bolts together components and positions them with set screws and then injects the epoxy.
One other variation of the idea, discussed in U.S. Pat. No. 4,863,149, uses a complex assembly fixture that has "arms" which position/hold tapered gib plates in position within the roller-ball cavities of an upside-down carriage while the epoxy is poured. The fixture has electromagnets that hold the gib plates to its "arms" while the arms are extended into the cavities. After the epoxy hardens, the magnets are released and the fixture "arms" are withdrawn. The gib plates are tapered and located within the bearing cavities, with bearing assemblies that are angled in a reverse-direction to slip into these cavities. Because of the opposing tapers, the depth at which the bearing assemblies sit (and thus their pre-load) can be adjusted by moving the bearing longitudinally within the cavity. A screw assembly is included along the longitudinal axis of each design.
Other applications of manufacture may be represented by U.S. Pat. No. 4,970,773 describing a method/apparatus for locating operational surfaces on a Mag-Lev Train. It could be viewed as analogous to attaching a long rail in proper position to a structure with respect to the train with the side rails being held in place by a fixture and a space existing between the inside of the side rail and the outside of the track being filled with concrete. Another similar patent is U.S. Pat. No. 5,065,489 which applies to multiple pieces in a copier machine; the pieces must be in good alignment to fit. During service of the machine, they are removed and must be realigned to facilitate reassembly. An "aligning member" is slipped over the already installed and aligned components, forming a cavity between the aligned components and the inside of the member, and the cavity is filled with adhesive, thereby making one integrated piece that can be removed and reinstalled easily since the adhesive holds the parts in the same relative position as when the adhesive was injected. Returning to bearing manufacture, the bearing manufacturing systems described in the preceding patents address methods for manufacturing rolling element bearing-based machines. These are, however, very different functionally and manufacturing-wise from the externally pressurized bearing assemblies of the present invention in which the flow through a hydrostatic bearing is reversed in order to hold it accurately in place against the bearing rail, and then the bearing is potted in place with epoxy. Upon restoring positive pressure, the bearing will then function normally. The vacuum method described herein provides indeed the highest level of even distributed force to attract the bearing surface to the bearing rail. This results in an exact matching of the surfaces in a low stress manner which helps to ensure the attainment of accuracy and long term stability.